The phylum Mollusca contains some of the most familiar invertebrates, including snails, slugs, clams, mussels, and octopuses. In contrast to these well-known molluscs, however, others are almost never seen, such as the aplacophorans and monoplacophorans, the latter of which which were only known from Paleozoic fossils until the first live specimen was discovered in the deep sea in 1952 (UCMP 2008).

Except for the aplacophorans, most molluscs have a well-developed, muscular foot. This structure is used in a multitude of ways, for example: locomotion, clinging to surfaces, burrowing, anchoring in sediment, swimming, and grasping (modified into prehensile tentacles in octopuses). The vast diversity of foot adaptations exemplifies the huge morphological diversity of the mollusc form.

A layer of epidermal tissue called the mantle surrounds the body of molluscs. Specialized glands in the mantle are responsible for the extracellular excretions that form shell structures. In all molluscan groups the shell is produced in layers of (usually) calcium carbonate, either in calcite or aragonite form.

Molluscs have adapted to terrestrial, marine and freshwater habitats all over the globe, although most molluscs are marine. Nearly 100,000 mollusc species are known (excluding the large number of extinct species known only as fossils) and it is clear that many thousands of species of extant species remain undescribed. Around 80% of known molluscs are gastropods (snails and slugs).

Despite the highly diverse forms of the members of this taxon, molluscs share a recognizable and characteristic generalized general body plan, made up of a head, a foot, and viscera contained in a central body. They are generally considered unsegmented, although primitive forms (aplacophorans and polyplacophorans) with repeated body features show intriguing potential for a possibly segmented mollusc-annelid ancestor (e.g. Jacobs et al 2000).

The mollusc head can house various combinations of sensory structures: tentacles, photoreceptors, statocysts, chemoreceptors. In some molluscs these sensory systems can be very well developed (the complex cephalopod eye is a prime example). Also found on the head is a feature unique to molluscs: the radula. Found in the buccal (mouth) cavity, the radula usually exists as a tongue-like plate covered with “teeth” used by herbivores, carnivores and scavengers to scrape food particles into the mouth. Depending on diet and use, tooth number, shape, arrangement, makeup, and growth have adapted diversely. Especially in the gastropods, number and shape of radular teeth are important taxonomic characters. The radula has also been adapted for diverse feeding methods. Some gastropods and cephalopods have a drill-like radula used to bore holes in the shell of prey, sometimes with the aid of acids secreted from an adjacent boring gland. In cone snails the radula is set on the end of a retractable proboscis and is slung out like a harpoon, to inject toxins into the prey, delivered through piercing, hollow teeth. In some cases these toxins are powerful neurotoxins, deathly to humans. Several lineages of molluscs have evolved suspension feeding, especially in the gastropods and bivalves. The radula in these cases is either highly reduced or lost altogether, and in most cases food particles are caught by ctinidia (gills) and moved to the mouth by cilia.

Except for the aplacophorans, most molluscs have a well-developed, muscular foot. This structure is used in a multitude of ways, for example: locomotion, clinging to surfaces, burrowing, anchoring in sediment, swimming, modified into prehensile tentacles (octopus); the vast diversity of foot adaptations exemplifies the huge morphological diversity of the mollusc form.

A layer of epidermal tissue called the mantle surrounds the body of molluscs. Specialized glands in the mantle are responsible for the extracellular excretions that form shell structures. The ancestral mollusc is thought to have one shell capped over the body like a limpet, and from that a diverse number of shell arrangements have evolved. Molluscs may have have one, two, or eight (in chitons) shells. Aplacophorans have no shell, but have instead minute aragonite spicules imbedded within the mantle. Secondary loss or much reduced shell vestiges have also occurred independently in multiple mollusc lineages (for example nudibranchs, slugs, cephalopods). Shells usually provide external protection, but there have been several independent internalizations within cephalopods and opisthobranchia. In all molluscan groups the shell is produced in layers of (usually) calcium carbonate, either in calcite or aragonite form. The wide range of pigmentation, shape, size, sculpturing, and twisting of sea shells is, of course, well known. There is much recent developmental work describing gene expression in shell formation, and the roles of highly conserved regulatory genes such as engrailed and Hox genes have been examined (e.g. Jacobs et al 2000, Samadi and Steiner 2009).

Between the mantle and the body proper is the mantle cavity, which may be organized as one or two separate spaces or grooves. Many important functions occur in the mantle cavity: the ctenidia (gills) are positioned here and the body systems, namely the nephridia (kidney like organs), the gut and the reproductive organs open up into this space. In aquatic molluscs cilia on the surface of the mantle and organs maintain water flow through the mantle cavity to take away wastes and bring in oxygenated water (and food particles for those suspension feeding molluscs). Molluscs have an open circulatory system with a full heart (with the exception of the cephalopods, which have a closed circulatory system). Their nervous system is well developed, usually consisting of a dorsal ganglion, a ring of nerves around the esophagus, and two pairs of lateral nerve cords running the length of the body, which are connected transversely in a ladder-like arrangement. There is an enormous range of nervous system development in the molluscs, from the poorly developed ganglia of the aplacophorans to the extreme cephalization of the cephalopods. Important work in the fields of neurobiology has been carried out on the squid Doryteuthis pealeii (formerly Loligo pealeii) and on Aplysia sea slugs.

Molluscs range in size from almost microscopic to animals 20 meters long (giant squid) or weighing 450 pounds (giant clams).

Molluscs have adapted to terrestrial, marine and freshwater habitats all over the globe, although most molluscs are marine.

Mollusca is prey of:
mixed-food consumers
secondary carnivores
Callinectes
Actinopterygii
Alburnus alburnus
bleak
Geococcyx californianus
Pollicipes polymerus
Huso huso
Oncorhynchus tshawytscha
Pseudodoras niger
Lates niloticus
Lepomis megalotis
Coris aygula
Ambystoma annulatum
Caretta caretta
Thamnophis butleri
Diadophis punctatus
Gavia stellata
Diomedea epomophora
Sula dactylatra
Egretta thula
Egretta tricolor
Mycteria americana
Eudocimus ruber
Cygnus olor
Anas fulvigula
Anas strepera
Anas cyanoptera
Anas americana
Aix sponsa
Aythya americana
Pandion haliaetus
Coturnix delegorguei
Actitis macularia
Larus canus
Fratercula cirrhata
Anodorhynchus hyacinthinus
Passerella iliaca
Corvus caurinus
Sorex gaspensis
Neurotrichus gibbsii
Peromyscus gossypinus
Lagenorhynchus australis
Lagenorhynchus cruciger
Feresa attenuata
Phocoenoides dalli
Delphinapterus leucas
Monodon monoceros
Mesoplodon europaeus
Mesoplodon carlhubbsi
Mesoplodon layardii
Enhydra lutris
Zalophus californianus
Neophoca cinerea
Callorhinus ursinus
Arctocephalus australis
Arctocephalus philippii
Arctocephalus townsendi
Phoca largha
Monachus tropicalis
Mirounga leonina
Mirounga angustirostris
Cephalophus niger
Alligator mississippiensis
Puma concolor
Prionailurus viverrinus
Mesoplodon peruvianus
Coturnix adansonii
Eremophila alpestris
Euoticus elegantulus
Cebus olivaceus
Hydromys chrysogaster
Ambystoma mexicanum
Amblonyx cinereus
Lontra provocax
Lutrogale perspicillata
Melogale personata
Martes melampus
Arvicola terrestris
Solenodon paradoxus
Potamogale velox

Based on studies in:
unknown: Black Sea (Marine)
Mexico: Guerrero (Coastal)
Uganda (Lake or pond)
England, River Thames (River)

This list may not be complete but is based on published studies.

Mollusca preys on:
detritus
phytoplankton
saprophagous plankton
algae
zooplankton
seston

Based on studies in:
unknown: Black Sea (Marine)
Mexico: Guerrero (Coastal)
England, River Thames (River)
Uganda (Lake or pond)

This list may not be complete but is based on published studies.

Animal / predator
adult of Muricidae is predator of Mollusca

Animal / parasite
larva of Sarcophaga melanura parasitises Mollusca
Other: minor host/prey

Like other systems, reproduction is highly variable among molluscs. Molluscs can be dioecious or simultaneously or sequentially hermaphroditic. Gametes are freely spawned in some groups, others have internal fertilization and complex mating behaviors, many produce egg capsules, egg cases, or brood chambers.

Most molluscs undergo spiral cleavage. Development can be direct (proceed right to settling into a juvenile form) or indirect, going through the swimming trochophore larval stage. The trochophore is very similar to the annelid trochophore. Before settling, many groups then go onto a second larval stage which is unique to molluscs: the feeding (usually) and swimming veliger larvae. Molluscs go through the uniquely molluscan process of torsion, usually during the veliger stage of development. Torsion involves counterclockwise rotation of the visceral mass up to 180 degrees with respect to the head and foot, to profoundly change the relative location of the body regions. Many groups then “detort” to some degree later in development or adulthood. Theories as to the evolutionary significance of torsion abound but this phenomenon is not well understood (Brusca and Brusca 2003). In the long run, torsion has allowed for much morphological diversification over the course of evolution.

The molluscs demonstrate remarkable morphological diversity, a characteristic that has confused molluscan taxonomy from the group’s inception. The Latin root molluscus means soft, and many soft-bodied invertebrates have been added and removed this group until Cuvier’s modern approximation in 1795 (Brusca and Brusca 2003). Mollusca is the second largest invertebrate phylum after the arthropods. Some 93,000 extant species have been described, but the thinking is this number represents only about half of the living species. 70,000 fossil species are also known. Most classifications recognize ten molluscan classes (two extinct). One class, the gastropods (snails and slugs), contains about 80% of mollusc species.

A very rich molluscan fossil record dates back 500 million year to the Precambrian. The evolutionary origins of molluscs are still disputed, but recent well-respected molecular phylogenetic analyses place the molluscs in the Lophotrochozoa, along with annelids, brachiopods, bryozoans and several other phyla (Halanych et al. 1995). Relationships within the Mollusca are also unclear and disputed; some recent analyses challenge whether this enormous phylum is a natural, monophyletic group (Sigwart and Sutton 2007 and references therein).

Shell protects, supports, and allows for growth: shelled mollusks
 

The shells of many mollusks provide protection and support while accomodating growth due to their conical structure.

           
  "Consider shapes that satisfy the following set of conditions. To provide both support and protection for the organism, the shape must be a hollow one, but an opening must exist somewhere. Growth can occur only by addition to the inner surface or the free edge. And the shape should change only minimally as it grows. A cubic shell with an open face won't work: addition to walls will give more shell relative to its contained volume, and addition to cylinder doesn't meet the conditions--addition to the edge will move it from short and fat to long and (relatively) thin. What will work are cones, whether circular or elliptical. Add to the edge and thicken the walls and one gets a bigger cone, isometric with the original.

With only slight variations of the condition of isometry, all sorts of wild derivatives of cones are possible--and these latter are the shapes in which shelled mollusks occur." (Vogel 2003:88-89)
  Learn more about this functional adaptation.

Protein plays role in crystal formation: molluscs
 

The conchiolin protein of many molluscs plays a role in shell formation by serving as a major matrix component for crystal formation.

   
  "The shell is secreted by the mantle, the tissue layer under the shell, of the mollusc, and consists of two or three layers. The outermost is the periostracum, made of a tough protein called conchiolin. The periostracum is often brown in colour although it may be so thin that it is virtually transparent: sometimes it is quite furry…Inside the periostracum are one or two layers of argonite or calcite, different crystalline forms of calcium carbonate, more commonly known as chalk. The main central layer is called the prismatic layer: the inner layer is known as the lamellate or nacreous layer. Here the crystals are laid in an overlapping zigzag formation that scatters light and produces the iridescent effect known as 'mother of pearl.'" (Foy and Oxford Scientific Films 1982:115)
  Learn more about this functional adaptation.

Collection Sites: world map showing specimen collection locations for Mollusca

Barcode of Life Data Systems (BOLD) Stats
                                                             

Specimen Records:	51,767
Specimens with Sequences:	41,223
Specimens with Barcodes:	37,714
Public Records:	28,656
Species:	7,589
Species With Barcodes:	6,278

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Non-marine molluscs appear to have a very high extinction rate. Lydeard et al (2004) list terrestrial and fresh water mollusc extinctions as about 40% of total recorded animal extinctions, far greater than marine molluscs.

Many different molluscs have been integrated into human culture since prehistoric times in a plethora of forms: shell money, jewelry and food, crop pests, and disease carriers (Schistosomiasis is a watersnail-born parasite that effects hundreds of millions of people in the world).